An inductive plasma torch, which is based on inductive coupling to an ionized gas at or near thermal equilibrium and is electrodeless and can be used with any gas desired, is useful for many applications including non-thermal destruction of hazardous waste material. The torch forms a plasma of ions, electrons and neutral particles which is used to act on materials, such as waste material, to dissociate the material so that other compounds or materials may be formed from it. In particular, the plasma torch is used to excite the ions, electrons and neutral particles to an elevated temperature as a result of collisions and ultraviolet radiation.
Forms of such devices which operate based on inductive coupling to an ionized gas are well known. An early version of a plasma torch included a quartz tube of high thermal shock resistance disposed inside and coaxial to an induction coil which excited the plasma. Despite use of a high thermal shock resistance material to form the quartz tube, the early plasma torch suffered from melt or thermal stress cracking of the quartz wall. Numerous efforts were made to overcome this problem, including adding more cooling to the outside of the quartz confinement wall and later on, utilizing double wall jacketed quartz tubes with water cooling. Despite such efforts, early plasma torches failed to contain higher power densities at atmospheric pressures. Consequently, the power input involved in larger diameter systems was limited because of the destruction of the quartz material due to thermal stress.
In later developed plasma torches, segmented copper cooling tubes were arranged in a protective cylinder or sheath, some of which incorporated singular tubes ("Induction Plasma Heater With High Velocity Sheath", TAFA Bulletin 26-D8, July 1968) while others utilized thick wall counterflow tubes (U.S. Pat. No. 4,431,901 to Hull). The TAFA and Hull torches each included a copper tube protector, quartz tube and exciter coil, arranged in order of increasing diameter. The copper tube protector was segmented with individual tubes to reduce its interference with the coupling of the electric field component of the electromagnetic field from the coil. Otherwise, a complete contiguous metal protector would have absorbed much or all of the energy which meant no plasma would have been generated. However, a drawback of these systems is that they continued to use quartz tubes for secondary plasma containment and structural strength to withstand the pressures of the vacuums created primarily for plasma ignitions which are usually done at low pressures to enhance plasma excitation. The quartz was not only very brittle and could crack from simple mechanical stress, but was subject to continued destruction from intermittent electrical discharges between the coil and tube protector or from ionization of the gas space between the coil and tube protector.
The above problems are enlarged with an increase in the excitation frequency of radio energy used and become more serious at a frequency greater than approximately 1 MHz. To overcome such problems, some systems placed the coil in deionized water but the problem of quartz destruction continued with undesirable ionizations occurring within the gas space between the quartz tube and the metal tube protector. With the electromagnetic field strength being the greatest near the coil, undesirable discharges were more likely near the coil if not inhibited or prevented. Both the Hull and TAFA systems suffered from the above problems. Whenever the quartz wall would break, water would enter the system, flooding the apparatus and extinguishing the plasma. Moreover, the copper metal tube protector was subjected to destruction when used in oxidizing or reactive atmospheres.
To decrease electrical discharging problems, which were in part the result of capacitive coupling by the electric field, the technique of reducing the applied excitation frequency was used. Operating at lower frequencies also reduced the charge buildup in the plasma. As a result, many high power, ambient pressure RF plasma torches were forced to operate at reduced frequencies, such as at 0.5 Mhz. A drawback of operating at lower frequencies was that the plasma pinch effect was minimized. At higher frequencies, the plasma pinching effect increases, that is, the plasma is further drawn back from the wall of the containment tube thereby lessening the thermal stress to the wall and allowing for higher input powers with advanced reliability.
Further attempts to overcome the above problems were made, including utilizing ceramic walls with coolant flow between (which is disclosed in U.S. Pat. No. 5,200,595 to Boulos), radioactively cooled protector walls, porous ceramic confinement, and ceramic containment protectors. However, the above devices were generally subject to the same thermal stresses as quartz and were difficult and costly to manufacture.
The present invention eliminates disadvantages of the prior art and more particularly, improves on the plasma confinement capabilities of the metal tube protector to produce a system which is reliable and robust even at frequencies greater than 1 Mhz.